1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which obtains information for medical diagnoses on the basis of a magnetic resonance signal emitted from a subject.
2. Description of the Related Art
MR angiography (MRA) is a magnetic resonance imaging method intended for arteries and veins. MRA includes a time of flight (TOF) method using a gradient echo (GRE) method, and a black-blood (BB) method using a fast spin echo (FSE) method for visualizing a blood vessel at low signal intensity. Recently, a susceptibility-weighted imaging (SWI) method which applies the susceptibility effect of veins has been devised (refer to the specification of U.S. Pat. No. 6,501,272).
A non-contrast TOF method is a typical example of a white-blood method. The non-contrast TOF method utilizes an in-flow effect, so that an artery with a high flow velocity close to an inflow part of a slab is visualized at high signal intensity. In this non-contrast TOF method, it is difficult to visualize turbulent parts, and peripheral blood vessels such as perforating branches are not easily visualized. In other words, arteries are principally visualized in the non-contrast TOF method.
Furthermore, when an image is taken with a T1W-based sequence using a paramagnetic contrast medium, blood vessels are visualized at high signal intensity, which means a WB method. In addition, an MRA method in which blood vessels show higher signal intensity than background tissues is widely referred to as the WB method here.
In the BB method, blood vessels show lower signal intensity than peripheral tissues. In the BB method, low blood flows are also visualized, and blood vessel walls are correctly visualized. It is also possible in the BB method to visualize the turbulent parts which are difficult to visualize in the TOF method. The FSE method was initially used in a sequence of the BB method, but is not used very widely due to the problem of image processing, or other problems. In the BB method, while both arteries and veins show low signal intensity, the arteries can be highlighted by setting an echo time (TE) slightly shorter. In addition, when an image is taken with a T2*W based sequence using the paramagnetic contrast medium, blood vessels are visualized at low signal intensity, which means the BB method.
In the BB method, peripheral tissues also show low signal intensity, and it is therefore difficult to separately visualize the blood vessels alone. For example, it is difficult to exclude air by minimum intensity projection (mimIP) in the BB method. The visualization of blood vessels in the WB method can be relatively easily carried out by, for example, maximum intensity projection (MIP).
The above-mentioned conventional MRA has advantages and disadvantages in both the WB method and the BB method, and these methods are suitably used in accordance with purposes. However, it has been difficult in both the WB method and the BB method to clearly visualize various structures of blood vessels.
On the other hand, methods for collecting magnetic resonance signals include a method in which a spin is dephased and a method in which a spin is rephased. These two collection methods are selectively used in accordance with purposes. Thus, information for medical diagnoses can be obtained on the basis of the magnetic resonance signals collected in one of these methods.
Another technique has been known in which information obtained on the basis of magnetic resonance signals collected in one of the methods is subjected to different processing to obtain useful information. For example, as an imaging method more sensitive to a change in the magnetic susceptibility owing to T2* enhancement, there has been proposed a technique which carries out phase emphasizing processing for an absolute value image reconstructed on the basis of magnetic resonance signals collected by rephase (refer to Magn. Reson. Med. 52:612-618, 2004).
However, there is a limit to information that can be obtained from the magnetic resonance signals collected by dephase or rephase, therefore it has been impossible to obtain information necessary to satisfactorily visualize, for example, blood vessels.